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In 10 hypothetical cities over a period of twenty weeks the number of deaths per week resulting from automobile accidents are given in the following table. \begin{tabular}{|l|l|l|l|l|l|} \hline \(\mathrm{X}=\) number of deaths & 0 & 1 & 2 & 3 & 4 \\ \hline \(\mathrm{f}=\) number of weeks when \(\mathrm{X}\) deaths occurred & 109 & 65 & 22 & 3 & 1 \\ \hline \end{tabular} Construct a Poisson distribution that fits this data. How do the frequencies predicted by this distribution compare with the observed frequency?

Short Answer

Expert verified
The Poisson distribution with a mean of 0.61 deaths per week is a good fit for the given dataset. The predicted frequencies for deaths per week resulting from automobile accidents, calculated using the Poisson distribution, are very close to the observed frequencies: 108 (0 deaths), 66 (1 death), 20 (2 deaths), 4 (3 deaths), and 1 (4 deaths).

Step by step solution

01

1. Calculate the mean number of deaths per week.

Let's denote the number of deaths per week as X and the number of weeks when X deaths occurred as f. To compute the mean number of deaths per week, we will use the following formula: Mean, \(\mu = \frac{\sum{X \cdot f}}{\sum{f}}\). Using the provided data: \(\mu = \frac{(0 \cdot 109) + (1 \cdot 65) + (2 \cdot 22) + (3 \cdot 3) + (4 \cdot 1)}{109 + 65 + 22 + 3 + 1} = \frac{0 + 65 + 44 + 9 + 4}{200} = \frac{122}{200}\) So, \(\mu = 0.61\).
02

2. Determine the Poisson distribution.

Since we have calculated the mean number of deaths per week (\(\mu = 0.61\)), we can now determine the Poisson distribution. The formula for the Poisson distribution is: \(P(X=k) = \frac{e^{-\mu} \cdot \mu^{k}}{k!}\) Where k is the number of deaths per week, e is the base of the natural logarithm (approximately 2.71828), and \(k!\) is the factorial of k.
03

3. Calculate the predicted frequencies using the Poisson distribution.

Now, we will use the determined Poisson distribution to calculate the predicted frequencies for the given data. The predicted frequency for each value of k can be found by using the formula: Predicted frequency = \(P(X=k) \cdot \sum{f}\) Considering that there are 200 weeks in total, we can calculate the predicted frequencies for each value of k from 0 to 4: \(P(X=0) = \frac{e^{-0.61} \cdot 0.61^{0}}{0!} = e^{-0.61}\) Predicted frequency of 0 deaths: \( e^{-0.61} \cdot 200\) \(P(X=1) = \frac{e^{-0.61} \cdot 0.61^{1}}{1!}\) Predicted frequency of 1 death: \(\frac{e^{-0.61} \cdot 0.61}{1} \cdot 200\) \(P(X=2) = \frac{e^{-0.61} \cdot 0.61^{2}}{2!}\) Predicted frequency of 2 deaths: \(\frac{e^{-0.61} \cdot 0.61^{2}}{2} \cdot 200\) \(P(X=3) = \frac{e^{-0.61} \cdot 0.61^{3}}{3!}\) Predicted frequency of 3 deaths: \(\frac{e^{-0.61} \cdot 0.61^{3}}{6} \cdot 200\) \(P(X=4) = \frac{e^{-0.61} \cdot 0.61^{4}}{4!}\) Predicted frequency of 4 deaths: \(\frac{e^{-0.61} \cdot 0.61^{4}}{24} \cdot 200\)
04

4. Compare the predicted frequencies with the observed frequencies.

We'll now compare the observed frequencies (given in the table) with the predicted frequencies calculated in step 3: Observed frequencies: 109 (0 deaths), 65 (1 death), 22 (2 deaths), 3 (3 deaths), 1 (4 deaths). Predicted frequencies (rounded to the nearest whole number): - 0 deaths: \( e^{-0.61} \cdot 200 \approx 108\) - 1 death: \(\frac{e^{-0.61} \cdot 0.61}{1} \cdot 200 \approx 66\) - 2 deaths: \(\frac{e^{-0.61} \cdot 0.61^{2}}{2} \cdot 200 \approx 20\) - 3 deaths: \(\frac{e^{-0.61} \cdot 0.61^{3}}{6} \cdot 200 \approx 4\) - 4 deaths: \(\frac{e^{-0.61} \cdot 0.61^{4}}{24} \cdot 200 \approx 1\) Comparing these values, we can see that the predicted frequencies for deaths per week resulting from automobile accidents are very close to the observed frequencies. This indicates that the Poisson distribution is a good fit for this dataset.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Mean Calculation in Poisson Distribution
When analyzing statistical data such as the number of deaths due to automobile accidents in different cities over a particular time, the first step is often to calculate the mean. The mean helps summarize the data, indicating the average number of occurrences (in this case, deaths) per time unit (week).For a Poisson distribution, the mean (\(\mu\)) is crucial because it predicts the distribution's shape. To compute the mean, use the formula:\[\mu = \frac{\sum{X \cdot f}}{\sum{f}}\]Where:
  • \(X\) represents the number of deaths.
  • \(f\) stands for the number of weeks that \(X\) deaths occurred.
In our dataset, with various numbers of weekly deaths (given by \(X\)) over many weeks (given by \(f\)), we plug into the formula:\[\mu = \frac{(0 \cdot 109) + (1 \cdot 65) + (2 \cdot 22) + (3 \cdot 3) + (4 \cdot 1)}{109 + 65 + 22 + 3 + 1} = \frac{0 + 65 + 44 + 9 + 4}{200} = \frac{122}{200}\]This calculation reveals a mean of approximately 0.61 deaths per week, setting the foundation for constructing the Poisson distribution.
Predicted Frequencies Using Poisson Distribution
Once the mean is determined, predicting the frequency of occurrences for different numbers of accidents in a Poisson distribution becomes feasible. This is useful for understanding how likely certain occurrences are over a fixed period.To predict these frequencies, we leverage the Poisson probability formula:\[P(X=k) = \frac{e^{-\mu} \cdot \mu^{k}}{k!}\]Where:
  • \(P(X=k)\) is the probability of observing \(k\) deaths in a week.
  • \(e\) is the mathematical constant approximately equal to 2.71828.
  • \(k!\) represents the factorial of \(k\).
By calculating this for each possible number of deaths (from 0 to 4 in this case), we obtain a set of probabilities which are then multiplied by the total number of observed weeks (200) to get the predicted frequencies:
  • 0 deaths: \(e^{-0.61} \cdot 200\)
  • 1 death: \(\frac{e^{-0.61} \cdot 0.61}{1} \cdot 200\)
  • 2 deaths: \(\frac{e^{-0.61} \cdot 0.61^{2}}{2} \cdot 200\)
  • 3 deaths: \(\frac{e^{-0.61} \cdot 0.61^{3}}{6} \cdot 200\)
  • 4 deaths: \(\frac{e^{-0.61} \cdot 0.61^{4}}{24} \cdot 200\)
These calculations provide a predicted count of weeks for each death rate, serving as a comparison point to observed data.
Observed Frequencies vs. Predicted Frequencies
The final, often most insightful step is comparing the observed frequencies with the predicted frequencies calculated from the Poisson distribution. This comparison shows how well the theoretical distribution matches real-world data, helping to verify its accuracy and reliability in describing the dataset. In our example:
  • Observed frequencies are the actual recorded number of weeks each number of deaths occurred: 109 weeks with 0 deaths, 65 with 1 death, 22 with 2 deaths, 3 with 3 deaths, and 1 with 4 deaths.
  • Predicted frequencies, rounded to whole numbers for easier analysis, are the Poisson model's expectations based on the calculated probabilities: approximately 108 weeks for 0 deaths, 66 for 1 death, 20 for 2 deaths, 4 for 3 deaths, and 1 for 4 deaths.
Comparing these, there's a slight difference in each category, but the numbers are very close. This closeness validates that the Poisson distribution is a suitable model for the data, accurately representing the likelihood of having different numbers of deaths each week.

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Most popular questions from this chapter

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